1. Field of the Invention
The present invention relates to image forming apparatuses including a multi-beam optical scanning apparatus.
2. Description of the Related Art
Electrophotographic image forming apparatuses typically form an electro latent image on an image carrier by irradiation with a light beam emitted from an optical scanning apparatus. A tandem image forming apparatus that forms a multi-color image includes a plurality of stations. The stations each include an image carrier. The stations individually form an electro latent image, and develop the electro latent images using toners having different colors to form toner images. The toner images having different colors are transferred, one on top of another, to a recording material or an intermediate transfer member. If the positions of the transferred toner images having different colors are misaligned, a color misalignment may occur that can be recognized by the naked eye.
A color misalignment also occurs at an end portion of an image in the main scanning direction if scanning lines have different lengths (magnifications) in the main scanning direction in each of the stations. Differences in the scanning line magnification between the stations may be caused by differences in temperature between the stations. Japanese Patent Laid-Open No. 2002-273931 proposes a technique of detecting the temperature of the optical scanning apparatus using a temperature sensor, and correcting the frequency of the image clock based on the detected temperature, in each of the stations. The document states that this technique reduces the differences in the magnification between the stations that are caused by the differences in temperature between the stations.
Japanese Patent Laid-Open No. 2002-273931 is directed to an image forming apparatus in which a single light beam is used for each color. In recent years, however, an optical scanning apparatus has been proposed in which one or more scanning lines are drawn by simultaneously outputting a plurality of light beams. Such an optical scanning apparatus is called a multi-beam optical scanning apparatus. In the multi-beam optical scanning apparatus, not only differences in the magnification between the stations (inter-station magnification differences), but also differences in the magnification between the light beams in the same station (inter-beam magnification differences), cause a problem. In other words, even if correction data for correcting the differences in the magnification between the stations is available, image moire cannot be reduced by only applying the correction data directly to the light beams.
In the multi-beam optical scanning apparatus, a drum-shaped image carrier is simultaneously irradiated with a plurality of light beams at separate positions in the sub-scanning direction. The image carrier has a circular cross-section, and therefore, the surface (circumferential surface) of the image carrier has a curvature factor. The optical paths of the light beams have lengths that vary depending on the curvature factor. Therefore, a difference in the magnification depending on the curvature factor occurs between the light beams. In other words, even for the same image data, scanning lines drawn by the light beams have different lengths (scan lengths).
The magnification differences between the light beams cause image moire due to a correspondence relationship with a screen process used in an image process. If a process of drawing a plurality of scanning lines using a plurality of light beams per scan is performed a plurality of times, a scanning line misalignment occurs periodically. If the periodic scanning line misalignments interfere with the screen process for binarization, image moire occurs. If the magnification of each of the light beams can be detected in real time, the magnification difference between the light beams can be corrected. To date, however, a pattern or sensor for performing such real-time detection has not been provided. In other words, only patterns and sensors for detecting relative magnifications between the stations have been previously proposed. Therefore, in order to reduce image moire, it is necessary to provide any technique of correcting the magnification, depending on the temperature, for each of a plurality of light beams in the same station.